Electric actuators such as a motor and a solenoid have been widely used for conversion of electric signals to mechanical motions or hydraulic pressure as various control targets are electronically controlled. It is essential to control a current with high accuracy to sophisticate the electric actuators.
For such high-accuracy current control, accurate detection of a current value is required. In general, the following method is used for this purpose: a total current is made to flow through a highly accurate shunt resistor to measure the voltage across the shunt resistor. In this method, however, the highly accurate and large-capacity shunt resistor is used, resulting in an increase in the cost and size of a control device. Further, since the total current to be detected flows through the shunt resistor, the amount of heat generated therefrom is large and thereby the cost for heat radiation will increase.
To solve the aforementioned problem, the following technique is known: a technique for detecting a current at low loss by connecting a main MOSFET for driving a, current to a current detector that uses a sense MOSFET in parallel (refer to, for example, Patent Document 1). A gate for the sense MOSFET is the same as that for the main MOSFET. The gate width of the sense MOSFET is designed approximately in a range of 1/1000 to 1/100. The amount of a current flowing in the sense MOSFET is approximately 1/1000 to 1/100 the amount of a current flowing in the main MOSFET. This makes it possible to lessen loss of a current that will occur in the current detector, compared with the method using the shunt resistor.